1. Field of the Invention
The present invention relates to electrophotographic printers and, more particularly, to so-called "cut paper electrophotographic printers" for printing sheets of cut paper with an electrophotographic process, each printer having an improved paper feed mechanism.
2. Background of the Invention
Before entering into the detailed description of the present invention, an electrophotographic printer according to the prior art will be described with reference to the schematic diagram of FIG. 1.
A photosensitive drum 1, a pair of feed rolls 2 and a pair of fixing rolls 3 rotate at a constant speed at all times. For printing operations, the photosensitive drum 1 is uniformly charged by a charging unit 6. Printing signals are transmitted from a host control circuit 11 to an optical unit 7 so that a laser beam 8 carrying the printing signal exposes the surface of the photosensitive drum 1. The portions exposed by the laser beam 8 are developed with toner by a developing unit 9. This toner is transferred to a sheet of paper 10 by a transfer unit 12 and is fixed on the paper 10 while passing through the fixing rolls 3.
The relation between the paper 10 and the laser beam 8 will be described in detail in the following in connection with the series process described above. A pair of timing rolls 5 are provided for arranging the top margin of the paper 10, i.e., the distance from the top edge to the first line. The paper 10, picked up by a pickup roll 4, is moved forward by the feed rolls 2 to come into abutment against the timing rolls 5. At this time these timing rolls 5 are held stopped. When the first line on the photosensitive drum 1 exposed by the laser beam 8 reaches a predetermined position, the timing rolls 5 are driven to send the paper 10 toward the transfer unit 12. Since the upper end and first line of the paper 10 have their phases arranged in this way, their distance is arranged for each sheet of paper 10.
Here arises a problem in that the temporary halt of the paper 10 results in a gap A (Note: Reference numeral "A" and other alphabetical reference numerals will be underlined throughout the specification to distinguish the reference numerals from word text) between the sheets of paper 10 as they cross the photosensitive drum 1, as is better seen from FIG. 2. This gap A, thus required, is different depending upon the printing rate of the printer, but is about 100 mm for a printer having a printing rate of about 50 sheets of A4 size (i.e., 210 mm.times.297 mm or 8.27 in..times.11.69 in.) paper per minute. This makes it necessary to increase the developing/transferring (i.e., processing) rate to an extent corresponding to the gap A. Specifically, the processing rate of the printer having the printing rate of about 50 sheets of A4 size (i.e., 210 mm.times.297 mm or 8.27 in..times.11.69 in.) paper per minute is calculated as follows:
(210 mm (per width of A4 paper).times.50 (sheets/minute)+100 mm (width per Gap A).times.50 (Gaps/minute)/60 sec (i.e. 60 sec./minute)=258 mm/sec.
If the gap A could be reduced to zero, the processing rate at this time would be calculated as follows and shortened to a factor of about 1/1.5:
(210 mm (per width of A4 paper).times.50 (sheets/minute)/60 sec (i.e., 60 sec./minute)=175 mm/sec.
On the contrary, the electrophotographic printer using a roll of continuous paper does not require the gap A so that it can enjoy a relatively higher printing rate. On the other hand, the transferring, developing and fixing techniques become more difficult for the higher processing rate. If the gap A can be reduced, it is accordingly expected that a cut paper electrophotographic printer having a higher printing rate can be produced.